DE 10 2009 028 542 A1 describes a method and a device for controlling a brake system. When using the device or when implementing the method, a portion of the brake system is hydraulically decoupled from the remaining portion of the brake system, upon which a pressure change in the decoupled portion of the brake system resulting from a volume displacement in the decoupled portion is ascertained and evaluated for the purpose of determining a p-V characteristic curve (pressure-volume characteristic curve). Controlling the brake system is to be facilitated by way of the obtained p-V characteristic curve of the decoupled portion of the brake system.